create a new column based on cumulative occurrences of a specific value in another column pandas

Question

I want to count the number of occurrences of one specific value (string) in one column and write it down in another column cumulatively.

For example, counting the cumulative number of Y values here:

col_1  new_col
Y        1
Y        2
N        2
Y        3
N        3

I wrote this code but it gives me the final number instead of cumulative frequencies.

df['new_col'] = 0
df['new_col'] = df.loc[df.col_1 == 'Y'].count()

Answer 1

To count both values cumulatively you can use:

df['new_col'] = (df
                 .groupby('col_1')
                 .cumcount().add(1)
                 .cummax()
                 )

If you want to focus on 'Y':

df['new_col'] = (df
                 .groupby('col_1')
                 .cumcount().add(1)
                 .where(df['col_1'].eq('Y'))
                 .ffill()
                 .fillna(0, downcast='infer')
                 )

Output:

  col_1  new_col
0     Y        1
1     Y        2
2     N        2
3     Y        3
4     N        3

Answer 2

df1.assign(new_col=df1.col_1.eq("Y").cumsum())

Output:

  col_1  new_col
0     Y        1
1     Y        2
2     N        2
3     Y        3
4     N        3